imx: Place imx_ddr_size() into a separate file

Place imx_ddr_size() into a separate file.

The motivation for doing this is to be able to easily reuse
imx_ddr_size() on i.MX7ULP.

Currently imx_ddr_size() is inside arch/arm/mach-imx/cpu.c, which
is not built for i.MX7ULP.

Changing the logic to allow building cpu.c for i.MX7UP would
require adding several ifdef's, leading to a not a very elegant
solution.

To allow better reuse, just place imx_ddr_size() into a common
mmdc_size.c file.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Stefano Babic <sbabic@denx.de>
diff --git a/arch/arm/mach-imx/Makefile b/arch/arm/mach-imx/Makefile
index fbd99a3..cb6ef53 100644
--- a/arch/arm/mach-imx/Makefile
+++ b/arch/arm/mach-imx/Makefile
@@ -21,6 +21,9 @@
 
 ifeq ($(SOC),$(filter $(SOC),mx5 mx6))
 obj-y	+= cpu.o speed.o
+ifneq ($(CONFIG_MX51),y)
+obj-y	+= mmdc_size.o
+endif
 obj-$(CONFIG_GPT_TIMER) += timer.o
 obj-$(CONFIG_SYS_I2C_MXC) += i2c-mxv7.o
 endif